Radio frequency (RF) and microwave filters and components (such as resonators, couplers, inductors, etc) are used in the wireless communication arts (for example, in cell phones and Wi-Fi) and in broadcast radio and television for filtering media signals at extremely high frequencies in the megahertz to gigahertz frequency ranges. Most devices that transmit or receive a signal, either by wire or through the air, include at least one such RF or microwave filter.
The complete RF spectrum includes the microwave band of frequencies at approximately 1-100 GHz (wavelength of 1 meters to 1 millimeters in package dielectric medium), so “RF filter” as used herein may include microwave filters, without explicitly being stated. Microwave filters are used in such technologies as radar, mobile and satellite communications, remote-sensing systems, measurement, and even electronic warfare, for example. Microwave filters perform the same function as RF filters in general, but circuit dimensions and the more exclusive use of distributed circuit elements instead of lumped-element capacitors and inductors reflect the filtering of higher frequency and shorter wavelengths in the microwave band as compared with other longer wavelength radio and communication signals.
Such RF and microwave filters may be used in components that either separate multiple frequency bands or combine the bands. Bandpass filters, for example, select only a desired band of frequencies out of a wider range, while band-stop filters eliminate an undesired band of frequencies. Lowpass filters allow only frequencies below a selected cutoff frequency to pass, while highpass filters allow only frequencies above a selected cutoff frequency to pass. Most RF and microwave filters are made up of one or more coupled resonators. The unloaded “quality factor” of the resonators used in a given filter determines how precisely it can select between frequencies. Thus, the operation of a microwave filter, for example, depends on resonant frequencies and the coupling coefficients of coupled resonators.
A “distributed element” type of RF filter has circuit elements that are not localized in discrete capacitors and inductors. The distributed elements are short lengths of the conducting circuit itself in various geometries and separated segments that cause a discontinuity in an applied AC signal. These discontinuities present a reactive impedance to a wavefront of the signal traveling down the line. The geometry of the distributed elements can be selected so that these “reactances” approximate inductors, capacitors, and resonators of a desired theoretical RF filter design. A resonant circuit, tank circuit, or tuned circuit is usually an LC circuit, the “L” representing one or more inductors, and the “C” representing one or more capacitors.
Distributed element RF filters often make use of stubs, which are geometrical side branches of the circuit, to emulate capacitors or inductors (determined by a stub's length, for example). Over a wide band, the stub can behave as a resonator. For example, an open-circuit quarter-wavelength stub behaves as a series LC resonator while a quarter-wavelength stub that is short-circuited to ground behaves as a shunt LC anti-resonator.
Coupled lines may also be used as distributed filter elements. Like stubs, coupled lines can act as resonators and likewise be terminated as short-circuited or open-circuited. Coupled lines tend to be preferred in planar technologies, where they are easy to implement. Theoretically, a true open circuit in planar technology is not ideally feasible because the dielectric effect of the substrate always maintains some small shunt capacitance providing some degree of short-circuit.
Microstrip conduction lines can also make good resonators for filters, as can stripline and coplanar waveguide (CPW) circuits, which are formed of a conductor separated from a pair of ground planes, all on the same plane atop a dielectric medium. The processes used to manufacture microstrip circuits are similar to the processes used to manufacture printed circuit boards and so this type of RF filter construction has the characteristic of being largely planar.
Distributed element RF and microwave filters, however, have the disadvantage of taking up much planar area or “real estate” on a motherboard or substrate, while filters made with discrete components are bulky in all three dimensions because of the components.
The proposed RF filter structures and methods described herein are also applicable to numerous other types of RF components, including resonators, couplers, inductors, capacitors, and so on.